Intervertebral prosthetic disc

ABSTRACT

A prosthetic disc for insertion between adjacent vertebrae includes upper and lower plates, a core disposed between the plates, and at least one projection extending from at least one of the upper and lower curved surfaces of the core into at least one recess of one of the inner surfaces of the plates. The recess is oversize with respect to the projection to allow sliding movement of the plate over the core while retaining the core between the plates during such sliding movement. The projection(s) may include a rod extending through an axial hole in the core, multiple surface features of the core, or the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/167,763 (Attorney Docket No. 29850-703.308), filed May 27, 2016, nowU.S. patent Ser. No. ______, which is a continuation of U.S. patentapplication Ser. No. 14/287,709, filed May 27, 2014, which is acontinuation of Ser. No. 12/759,460 filed Apr. 13, 2010, which is acontinuation of Ser. No. 12/101,664 filed Apr. 11, 2008, whichapplication is a continuation of Ser. No. 10/855,817 filed May 26, 2004(now U.S. Pat. No. 7,442,211), which is a non-provisional of U.S.Provisional Application Nos. 60/473,802 and 60/473,803, both of whichwere filed May 27, 2003; the full disclosures of which are herebyincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to medical devices and methods. Morespecifically, the invention relates to a prosthetic disc forintervertebral insertion, such as in the lumbar and cervical spine.

In the event of damage to a lumbar or cervical intervertebral disc, onepossible surgical treatment is to replace the damaged disc with a discprosthesis. Several types of intervertebral disc prostheses arecurrently available. For example, one type of intervertebral discprosthesis is provided by Waldemar Link GmbH & Co under the trademarkLINK® SB Charite. This prosthesis includes upper and lower prosthesisplates or shells which locate against and engage the adjacent vertebralbodies, and a low friction core between the plates. The core has upperand lower convexly curved surfaces and the plates have corresponding,concavely curved recesses which cooperate with the curved surfaces ofthe core. This allows the plates to slide over the core to allowrequired spinal movements to take place. The curved recesses in theplates are surrounded by annular ridges which locate, at the limit ofsliding movement of the plates over the core, in opposing upwardly anddownwardly facing, peripheral channels surrounding the curved surfacesof the core.

This type of disc configuration is described in EP 1142544A1 and EP1250898A1, assigned to Waldemar Link GmbH & Co. A drawback of suchconfigurations is that the provision of the peripheral ribs and channelslimits the areas available for bearing and sliding contact between theplates and core, and accordingly the loads which can be transmitted bythe prosthesis. As a result of the relatively small bearing areas, it isbelieved that at least the core will be subject to rapid wear and have arelatively short lifespan. Also, because the core is in effect merely“clamped” between the plates, this configuration does not allow forsecure retention of the core. In one alternative arrangement, the curvedsurfaces of the core carry opposing, elongate keys that locate inelongate grooves in the plates and another alternative arrangement inwhich the plates have opposing elongate keys that locate in elongategrooves in the opposite curved surfaces of the core. These key andgroove arrangements allow the plates to slide over the core within thelimits of the length of the grooves, in one direction only. Althoughallowance is made for some lateral play of the keys in the grooves, verylittle sliding movement of the plates over the core can take place inthe orthogonal vertical plane, and this is considered to be a seriousdrawback of this design.

Other currently available intervertebral disc prostheses have similarand/or other drawbacks. Typically, drawbacks include insufficientresistance to wear and tear, restricted range of motion and/orinsufficient ability of the prosthesis to adhere to vertebral bone.

Therefore, a need exists for improved intervertebral disc prostheses.Ideally, such improved prostheses would resist wear and tear, provide adesired range of motion and adhere well to vertebral bone. At least someof these objectives will be met by the present invention.

2. Description of the Background Art

Published US patent applications 2002/0035400A1 and 2002/0128715A1describe disc implants which comprise opposing plates with a corebetween them over which the plates can slide. The core receives one ormore central posts, which are carried by the plates and which locate inopposite ends of a central opening in the core. Such arrangements limitthe load bearing area available between the plates and core.

Other patents related to intervertebral disc prostheses include U.S.Pat. Nos. 4,759,766; 4,863,477; 4,997,432; 5,035,716; 5,071,437;5,370,697; 5,401,269; 5,507,816; 5,534,030; 5,556,431; 5,674,296;5,676,702; 5,702,450; 5,824,094; 5,865,846; 5,989,291; 6,001,130;6,022,376; 6,039,763; 6,139,579; 6,156,067; 6,162,252; 6,315,797;6,348,071; 6,368,350; 6,416,551; 6,592,624; 6,607,558 and 6,706,068.Other patent applications related to intervertebral disc prosthesesinclude U.S. Patent Application Publication Nos.: 2003/0009224;2003/0074076; 2003/0191536; 2003/0208271; 2003/0135277; 2003/0199982;2001/0016773 and 2003/0100951. Other related patents include WO01/01893A1, EP 1344507, EP 1344506, EP 1250898, EP 1306064, EP 1344508,EP 1344493, EP 1417940, EP 1142544, and EP 0333990.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the present invention, an intervertebral prostheticdisc for insertion between adjacent vertebrae comprises: upper and lowerplates having outer surfaces locatable against the respective vertebraeand inner, curved surfaces; a core between the plates, the core havingupper and lower curved surfaces complementary in shape to the inner,curved surfaces of the plates to allow the plates to slide over thecore; and at least one projection extending from at least one of theupper and lower curved surfaces of the core into at least one recess ofone of the inner surfaces of the plates, the recess being oversize withrespect to the projection to allow sliding movement of the plate overthe core while retaining the core between the plates during such slidingmovement.

Some embodiments further include multiple projections extending from theupper and lower surfaces of the core. For example, the multipleprojections may include two elevated rings projecting from a peripheralportion of each of the upper and lower surfaces of the core intoring-shaped recesses on the upper and lower plates. In otherembodiments, the multiple projections may comprise multiple surfacefeatures projecting from a peripheral portion of each of the upper andlower surfaces of the core into multiple recesses on the upper and lowerplates. In yet other embodiments, the multiple projections may compriserespective ends of an elongate, upright element extending axiallythrough the core, the ends projecting beyond the upper and lower coresurfaces. For example, the upright element may comprise a rod extendingthrough an axial passage through the core. In some embodiments, such arod and passage may be complementarily threaded for engagement with oneanother.

In some embodiments, the disc further includes at least one finextending from each of the outer surfaces of the plates to promoteattachment of the plates to the vertebrae. In some embodiments, each finextends from its respective outer surface at a 90.degree. angle. Inother embodiments, each fin extends from its respective outer surface atan angle other than 90.degree.. In some embodiments, each fin includesat least one hole for promoting attachment of the plates to thevertebrae. For further promoting attachment of the plates to thevertebrae some embodiments include outer surfaces of the plates that aretextured. For example, in some embodiments the textured surfacescomprise multiple serrations.

The plates may have any of a number of different configurations, sizes,or the like. In one embodiment, the outer surfaces of the plates areflat. In one embodiment, lateral edge portions of the upper and lowerplates are adapted to contact one another during sliding movement of theplates over the core.

In another aspect of the present invention, an intervertebral prostheticdisc for insertion between adjacent vertebrae comprises: upper and lowerplates having outer surfaces locatable against the respective vertebraeand inner, curved surfaces, at least one of the inner surfaces having atleast one recess; a core between the plates, the core having upper andlower curved surfaces complementary in shape to the inner, curvedsurfaces of the plates to allow the plates to slide over the core, andan axial passage extending through the core; and a rod extending throughthe axial passage into the at least one recess in the inner surface(s)of the plate(s). The recess are oversize with respect to the projectionto allow sliding movement of the plate over the core while retaining thecore between the plates during such sliding movement.

Optionally, the rod and passage may be complementarily threaded forengagement with one another. In some embodiments, the rod is movablyengaged with a first oversized recess on the upper plate and a secondoversized recess on the lower plate. In various embodiments, the platesand core may have any of the features or characteristics describedabove.

In another aspect of the invention, an intervertebral prosthetic discfor insertion between adjacent vertebrae includes: upper and lowerplates having outer surfaces locatable against the respective vertebraeand inner, curved surfaces; a core between the plates, the core havingupper and lower curved surfaces complementary in shape to the inner,curved surfaces of the plates to allow the plates to slide over thecore; and a flexible tie member extending laterally through the core andhaving ends outside the core which are engaged with one or both of theplates to retain the core between the plates when the plates slide overthe core. The flexible tie member, for example, may extend through alateral passage through the core and may include ends engaged with atleast one of the upper and lower plates. In some embodiments, theflexible tie member comprises a flexible cable or cord.

In yet another example of the present invention, an intervertebralprosthetic disc for insertion between adjacent vertebrae comprises:upper and lower plates having textured outer surfaces locatable againstthe respective vertebrae, each of the outer surfaces having at least onevertical fin and an edge portion adapted to contact a corresponding edgeportion of the other plate, and inner, curved surfaces; and a corebetween the plates, the core having upper and lower curved surfacescomplementary in shape to the inner, curved surfaces of the plates toallow the plates to slide over the core. The curved surfaces of theplates and core include formations which cooperate with one another toretain the core between the plates when the plates slide over the core.The formations include recesses and projections received by therecesses, and the recesses and projections are located between a centralaxis of the relevant curved surface and an outer periphery thereof.

In some embodiments, for example, the projections may comprise twoelevated rings projecting from a peripheral portion of each of the upperand lower surfaces of the core into ring-shaped recesses on the upperand lower plates. In other embodiments, the projections may comprisemultiple surface features projecting from a peripheral portion of eachof the upper and lower surfaces of the core into multiple recesses onthe upper and lower plates. Again, the plates and core may include anyof the features described above.

These and other aspects and embodiments are described more fully belowwith reference to the drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional anterior view of a prosthetic discaccording to one embodiment of the invention, with the prosthesis platesand core in vertical alignment;

FIG. 2 shows a cross-sectional side view of the disc of FIG. 1, aftersliding movement of the plates over the core;

FIG. 3 shows a cross-sectional side view of the disc of FIG. 1, aftertranslational movement of the plates relative to the core;

FIG. 4 shows a cross-sectional side view of the disc of FIG. 1, with theplates and core in vertical alignment;

FIG. 5 shows a plan view of the core of the disc of FIG. 1;

FIG. 6 shows a plan view of the upper plate of the disc of FIG. 1;

FIG. 6A shows a plan view of a disc, as in FIGS. 1 and 6, with a finrotated away from the anterior-posterior axis;

FIG. 7 shows a cross-sectional anterior view of a prosthetic discaccording to another embodiment of the invention with a flexible tiemember engaged with one plate and FIG. 7A shows a cross-sectionalanterior view of another prosthetic disc with a flexible tie memberengaged with both plates;

FIG. 8 shows a cross-sectional side view of the prosthetic disc of FIG.7;

FIG. 9 shows a cross-sectional anterior view of a prosthetic discaccording to another embodiment of the invention;

FIG. 10 shows a cross-sectional side view of the prosthetic disc of FIG.9; and

FIG. 11 shows a cross-sectional side view of another embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-4 illustrate a prosthetic disc 10 for intervertebral insertionbetween two adjacent spinal vertebrae (not shown). The disc 10 comprisesthree components, namely an upper plate or shell 12, a lower plate orshell 14 and a core 16 located between the plates.

The upper plate 12 includes an outer surface 18 and an inner surface 24and may be constructed from any suitable material or combination ofmaterials, such as but not limited to cobalt chrome molybdenum, titanium(such as grade 5 titanium) and/or the like. In one embodiment, typicallyused in the lumbar spine, the upper plate 12 is constructed of cobaltchrome molybdenum, and the outer surface 18 is treated with aluminumoxide blasting followed by a titanium plasma spray. In anotherembodiment, typically used in the cervical spine, the upper plate 12 isconstructed of titanium, the inner surface 24 is coated with titaniumnitride, and the outer surface 18 is treated with aluminum oxideblasting. An alternative cervical spine embodiment includes no coatingon the inner surface 24. In some embodiments, it may be useful to coupletwo materials together to form the inner surface 24 and the outersurface 18. For example, the upper plate 12 may be made of anMRI-compatible material, such as titanium, but may include a hardermaterial, such as cobalt chrome molybdenum, for the inner surface 24.Any suitable technique may be used to couple materials together, such assnap fitting, slip fitting, lamination, interference fitting, use ofadhesives, welding and/or the like. Any other suitable combination ofmaterials and coatings may be employed in various embodiments of theinvention.

In some embodiments, the outer surface 18 is planar. Oftentimes, theouter surface 18 will include one or more surface features and/ormaterials to enhance attachment of the prosthesis 10 to vertebral bone.For example, the outer surface 18 may be machined to have a serrations20 or other surface features for promoting adhesion of the upper plate12 to a vertebra. In the embodiment shown (FIG. 6), the serrations 20extend in mutually orthogonal directions, but other geometries wouldalso be useful. Additionally, the outer surface 18 may be provided witha rough microfinish formed by blasting with aluminum oxidemicroparticles or the like. In some embodiments, the outer surface mayalso be titanium plasma sprayed to further enhance attachment of theouter surface 18 to vertebral bone.

The outer surface 18 may also carry an upstanding, vertical fin 22extending in an anterior-posterior direction. The fin 22 is pierced bytransverse holes 23. In an alternative embodiment, as shown in FIG. 6A,the fin 22 may be rotated away from the anterior-posterior axis, such asin a lateral-lateral orientation, a posterolateral-anterolateralorientation, or the like. In some embodiments, the fin 22 may extendfrom the surface 18 at an angle other than 90.degree.. Furthermore,multiple fins 22 may be attached to the surface 18 and/or the fin 22 mayhave any other suitable configuration, in various embodiments. In someembodiments, such as discs 10 for cervical insertion, the fins 22, 42may be omitted altogether.

The lower plate 14 is similar to the upper plate 12 except for theabsence of the peripheral restraining structure 26. Thus, the lowerplate 14 has an outer surface 40 which is planar, serrated andmicrofinished like the outer surface 18 of the upper plate 12. The lowerplate 14 optionally carries a fin 42 similar to the fin 22 of the upperplate. The inner surface 44 of the lower plate 14 is concavely,spherically curved with a radius of curvature matching that of the innersurface 24 of the upper plate 12. Once again, this surface may beprovided with a titanium nitride or other finish.

The core 16 of the disc 10 is made of a low-friction material, such aspolyethylene (Chirulen™). In alternative embodiments, the core 16 maycomprise any other suitable material, such as other polymers, ceramicsor the like. For wear resistance, the surface zones of the core 16 maybe hardened by an appropriate cross-linking procedure. A passage 32extends axially through the core. The passage is provided with aninternally threaded sleeve 33 of titanium or other suitable material. Anelongate element in the form of a round cross-section, threaded rod 34extends axially through the passage and is in threaded engagement withthe sleeve 33. The length of the rod is greater than the axial dimensionof the core, with the result that the opposite ends 36 of the rodproject from the curved surfaces 28 and 30 of the core. In the assembleddisc 10, these ends 36 locate in the recesses 26. The diameter of therod is less than that of the recesses 26 so there is substantial roomfor the rod ends to move laterally in the recesses.

In use, the disc 10 is surgically implanted between adjacent spinalvertebrae in place of a damaged disc. The adjacent vertebrae areforcibly separated from one another to provide the necessary space forinsertion. The disc is inserted, normally in a posterior direction, intoplace between the vertebrae with the fins 22, 42 of the plates 12, 14entering slots cut in the opposing vertebral surfaces to receive them.After insertion, the vertebrae, facets, adjacent ligaments and softtissues are allowed to move together to hold the disc in place. Theserrated and microfinished surfaces 18, 40 of the plates 12, 14 locateagainst the opposing vertebrae. The serrations 20 and fins 22, 42provide initial stability and fixation for the disc 10. With passage oftime, enhanced by the titanium surface coating, firm connection betweenthe plates and the vertebrae will be achieved as bone tissue grows overthe serrated surface. Bone tissue growth will also take place about thefins 22, 40 and through the transverse holes 23 therein, furtherenhancing the connection which is achieved.

Referring to FIG. 5, the core 16 may be formed with narrow, angularlyspaced, blind passages 61 which accommodate titanium pins 64. In manyembodiments, the core 16 itself is transparent to X-radiation and so isinvisible in a post-operative X-ray examination. The pins 64 serve asradiographic markers and enable the position of the core 16 to beascertained during such examination.

In the assembled disc 10, the complementary and cooperating sphericalsurfaces of the plates and core allow the plates to slide or articulateover the core through a fairly large range of angles and in alldirections or degrees of freedom, including rotation about the centralaxis 40. FIGS. 1 and 4 show the disc 10 with the plates 12, 14 and core16 aligned vertically with one another on the axis 40. FIG. 2illustrates a situation where maximum anterior flexion of the disc hastaken place. Such flexion is enabled by the ability of the ends 36 ofthe rod to move laterally in all directions and through a fairly largedistance, in the recesses 26. At the position of maximum flexion, theends 36 of the rod abut the sides of the recesses as illustrated. At thesame time, the plates 12, 14 abut one another at the periphery of theircurved surfaces. Similar principles apply to maximum posterior flexureof the plates 12, 14 over the core, i.e. during spinal extension and/orin the event of maximum lateral flexure.

FIG. 3 illustrates how the disc 10 can also allow for translationalmovement of the plates relative to the core. In the illustratedsituation there has been lateral translation of the plates relative tothe core. The limit of lateral translation (not shown) is again reachedwhen the ends 36 of the rod abut laterally against the sides of therecesses 26.

In each case, the cooperating retaining formations, i.e. the ends 36 ofthe rod and the recesses 26 cooperate with one another to preventseparation of the core from the plates. In other words, the cooperationof the retaining formations ensures that the core is held captivebetween the plates at all times during flexure of the disc 10. In otherembodiments of this version of the invention, the rod can be mountedfixedly to the core by means other than the illustrated threadedconnection. In other embodiments, the rod may be replaced by separateelements projecting respectively from the upper and lower curvedsurfaces of the core.

FIGS. 7 and 8 illustrate another embodiment of the invention. In thisembodiment, the core 16 is formed with a lateral passage 50 extendingdiametrically through it. The passage is provided with a sleeve 52 oftitanium or other suitably wear resistant material. A flexible tiemeans, in this embodiment in the form of a cable 54 of braided titaniumconstruction, passes with clearance through the sleeve 52. The ends ofthe cable 54 are flexed upwardly and enter passages 56 in the upperplate 12. The extremities of the cable carry crimped retention lugs orferrules 58 anchored in blind ends of the passages 56. The flexible tiemember 54 extends laterally through the core 16 and has ends outside thecore which are engaged with one (FIG. 7) or both (FIG. 7A) of the plates12, 14 to retain the core between the plates when the plates slide overthe core.

The cable 54 holds the core 16 captive during sliding movement of theplates 12,14 over the core, whether in flexion, extension ortranslation. The cable can flex through a wide range of angles to allowsliding movement or articulation of the plates relative to the core totake place. The slack in the cable also allows a degree of rotationalmovement of the plates relative to the core. As illustrated in FIG. 7,the ends of the passage 50 and sleeve 52 are belled to accommodatemovements of the cable during sliding movements. Also, surfaces 60 ofthe plates 12, 14 are inclined to accommodate the cable when sliding hastaken place, so that the cable does not act directly on the plates.

FIGS. 9 and 10 illustrate another embodiment of a prostheses 10. In thisembodiment, the curved surfaces 24 of the plates 12, 14 are formed, atpositions between the central axis and their peripheries, withcontinuous, inwardly directed ribs 62 of annular shape. These ribslocate, with considerable clearance, in annular channels 64 provided atcorresponding positions in the upper and lower curved surfaces of thecore 16. Once again, cooperation between the retaining formations, i.e.the ribs and channels, holds the core captive between the plates whenthe plates slide over the core during flexion, extension or translation.At the limit of sliding movement in each case, the rib 62 will abutagainst a side of the channel. The channel may be provided with a wearresistant lining as described previously.

FIG. 11 illustrates another embodiment of a prosthesis. In this case,the core carries continuous, annular ribs 70 on its upper and lowersurfaces which locate with clearance in channels 72 in the plates 12,14. The ribs 70 may be lined with wear resistant material as describedpreviously.

In each of the later versions, i.e. those of FIGS. 7 to 11, the core 16may be provided with radiographic markers as described previously. Also,in each of these versions, the outer surfaces of the plates 12, 14 mayhave the same configuration as described in relation to the firstversion of FIGS. 1 to 6.

In FIGS. 1-6 and 9-11, embodiments are illustrated in which retainingformations are provided that cooperate with one another between bothplates and the core. In other embodiments, core retention may beachieved by cooperation between retaining formations which only actbetween one of the plates, either the upper plate 12 or the lower plate14, and the core. In one embodiment, for example, there may be a singleprojection, which extends from the upper (or lower) curved surface ofthe core and a corresponding recess in the inner surface of the lower(or upper) plate.

Although the foregoing is a complete and accurate description of theinvention, any of a number of modifications, additions or the like maybe made to the various embodiments without departing from the scope ofthe invention. Therefore, nothing described above should be interpretedas limiting the scope of the invention at it is described in the claims.

What is claimed is:
 1. A method of implanting a prosthetic disc, themethod comprising: providing a prosthetic disc comprising: upper andlower plates and a core, wherein the core has at least one convexbearing surface configured to slide over a corresponding bearing surfaceof one of the upper and lower plates upon implantation between adjacentvertebrae; providing a rib on only one of the upper and lower plates anda channel in a corresponding bearing surface of the core, with the riblocated between a central axis of the plate and an outer peripherythereof; positioning the core between the upper and lower plates withthe core held captive between the plates while the plates slide over thecore by locating the rib within the channel; forcibly separatingadjacent vertebrae from one another to provide space for insertion ofthe prosthetic disc; and inserting the prosthetic disc between theseparated vertebrae.
 2. The method of claim 1, wherein the upper andlower plates are allowed to slide over the core.
 3. The method of claim1, wherein the core comprises at least one spherical surface.
 4. Themethod of claim 1, wherein the rib extends into the core and does notextend through the core.
 5. The method of claim 1, wherein the ribextends outward from the at least one of the upper and lower plates in adirection toward the other one of the plates.
 6. The method of claim 1,wherein the core slides on a bearing surface of the at least one plateand the rib extends from the bearing surface toward the other one of theplates.
 7. The method of claim 1, wherein at least one of the upper andlower plates is free to rotate with respect to the core.
 8. The methodof claim 1, wherein the bearing surface of the plate at least partiallysurrounds the rib extending from the bearing surface.
 9. The method ofclaim 1, wherein the core comprises a polyethylene, polymer or ceramicmaterial.
 10. The method of claim 1, wherein a central axis of each ofthe upper and lower plates is located midway between lateral edges ofthe upper or lower plate; and wherein the rib is located between thecentral axis of the one of the upper and lower plates and an outerperiphery thereof
 11. The method of claim 1, wherein the core is asingle monolithic core.
 12. The method of claim 1, wherein the core hasupper and lower surfaces complementary in shape to the inner bearingsurfaces of the upper and lower plates to allow the upper and lowerplates to slide over the core upon implantation between the adjacentvertebrae.
 13. The method of claim 1, wherein the channel is located ina spherically curved surface of the core.
 14. The method of claim 1,wherein the rib has side surfaces located substantially at right anglesto the bearing surface on which the rib is located.
 15. The method ofclaim 1, wherein the rib is located with clearance in the channel. 16.The method of claim 15, wherein at a limit of sliding movement of theplates over the core, the rib abuts against the channel.
 17. The methodof claim 14, wherein the rib side surfaces include two opposite sidesurfaces which are substantially parallel.
 18. The method of claim 17,wherein the plate bearing surface at least partially surrounds the twoopposite side surfaces of the rib.
 19. The method of claim 1, whereinthe convex bearing surface of the core allows the core to rotate freelywith respect to at least one of the upper and lower plates.
 20. Themethod of claim 1, wherein the rib and channel do not intersect acentral axis of the core when the upper and lower plates and core arealigned vertically with one another.
 21. The method of claim 1, whereinthe rib is continuous rib.
 22. The method of claim 1, wherein the corehas a substantially circular shaped perimeter.
 23. The method of claim1, wherein the prosthetic is inserted between the separated vertebrae ina posterior direction.
 24. The method of claim 1, wherein afterinsertion of the prosthetic disc, the vertebrae, adjacent ligaments andsoft tissues are allowed to move together to hold the prosthetic disc inplace.
 25. The method of claim 1, wherein the rib and channel provide aretaining structure which restrains both lateral and anterior/posteriormovement of the core with respect to the upper and lower plates.